The invention relates to a polarizing beam splitter for splitting an incident non-collimated light beam having a given wavelength into two sub-beams having mutually different states of polarization, which polarizing beam splitter has a substrate provided with a polarizing stack of thin layers. The invention also relates to a magneto-optic reading device provided with such a beam splitter.
Polarizing beam splitters are suitable, inter alia for splitting an incident light beam into two sub-beams which are mutually perpendicularly polarized. Special polarizing beam splitters referred to as partially polarizing beam splitters (PPBS) split an incident light beam into two sub-beams each having a desired proportion of the quantity of light in each of the two perpendicular directions of polarization.
A polarizing beam splitter of the type described in the opening paragraph is known from European Patent Application no. 0 336 334. This beam splitter comprises a polarizing stack of thin layers alternately having a high and a low refractive index. The first and the last layer of the stack are thinner than the intermediate layers in order to realise a correct beam splitting ratio for light beams which are incident at angles deviating from the angle of incidence for which the beam splitter has been designed. Such a beam splitter may be positioned directly in a diverging light beam as emitted by a semiconductor diode laser. This eliminates the necessity of positioning a lens in front of the beam splitter for collimating the beam and leads to a more compact optical arrangement.
The use of a polarizing beam splitter as a beam separator in a magneto-optic reading device is known, inter alia from Japanese Patent Application no. 2-64917 (A). In this device a diverging light beam having a first direction of polarization generated by a diode laser is incident through the beam splitter on a lens system which focuses the beam on an information storage medium. The state of polarization of the beam reflected by the medium is modulated by the information stored in the medium so that this beam does not only comprise a reasonably large quantity of light having said first direction of polarization but also a small, varying quantity of light having a second direction of polarization perpendicular to the first direction. The lens system converges the reflected beam. The beam splitter reflects a part of the beam reflected by the medium towards a detection system, which converts the modulation of the beam into an electric detection signal representing the information which has been read. The beam splitter reflects all the light of the second direction of polarization and a part of the light of the first direction of polarization, so that the detected signal is amplified. However, it appears that the use of the beam splitter known from said European Patent Application leads to a non-optimal detection signal in the reading device.